Exercise $4\mathrm{.}58$ Implement the number Instock method. This should locate

a product in the collection with a matching ID and return the current

quantity of that product as a method result. If no product with a matching

ID is found, return zero. This is relatively simple to implement once the

findProduct method has been completed. For instance, number Instock

can call the findProduct method to do the searching and then call the

getQuantity method on the result. Take care with products that cannot be

found, though.